Thin film sensors based on optical measurements typically measure the average thin film thickness over a given area. Two common optical approaches for the measurement of thin film thickness are based on ellipsometry and surface plasmon resonance. Other optical approaches are based on interferometry using two waveguide branches. An example of a non-optical thin film sensor is a quartz resonator where the film thickness is determined by measuring the change in mechanical resonance. Applications for the measurement of thin film thickness include the monitoring of fabrication processes and kinetic antigen-antibody experiments where the rate of deposition or adsorption is important. For typical optical sensor approaches, good sensitivity is achieved by measuring the angle of incident or reflected light with high precision. Typically, precise angle measurement in Surface Plasmon Resonance (SPR) requires either a large measurement area or large distances between the sample and the detector or both. To measure a very thin film it is typically necessary for the optical field to be tightly confined in the vertical dimension where the vertical dimension corresponds to the film thickness direction.